In recent years, powder metallurgy has been gaining increasing attention in efforts to obtain new microstructures and improved mechanical properties in alloys of aluminum. In adapting powder metallurgy to the manufacture of wrought products, the idea has been one of consolidating a powder to form a basic workpiece which often corresponds to the usual ingot or billet. Ideally, the workpiece can then be hot worked according to conventional methods to produce the desired wrought mill product, such as an extrusion, or forging or sheet.
To produce a workpiece of suitable quality, it has been recognized that the powder should be consolidated in a closely controlled manner. Care is needed, for example, to minimize internal workpiece contamination by oxidation, hydration or other reactions with extraneous elements or compounds. Further, in the known art of processing aluminum powder alloys is the stringent care that has been exercised in order to minimize internal workpiece porosities, particularly porosities which trap gases at any significant pressure.
A problem which is unique to aluminum powder metallurgy is the inevitable formation of oxides on the powder. These oxides form as thin films, on the order of several nanometers in thickness, on the surfaces of the individual particles. Unlike the oxides which form on other metals used in powder metallurgy, such as copper, iron and their alloys, the oxide film on aluminum or magnesium cannot be reduced to metal in situ. The oxide film on aluminum and its alloys consists almost exclusively of aluminum and magnesium oxides and their hydrates. These aluminum or magnesium oxide films inhibit the particle-to-particle bonding necessary in forming both good compacts and final products of acceptable ductility and toughness as well as strength. Thus, the formation of aluminum powder products is far more difficult and technically completely distinct from the powder metallurgy of metals with in-situ reducible oxides, such as copper, iron and their alloys.
According to a common prior art practice, consolidation of aluminum powders is carried out in the following manner. First, a porous compact is formed by cold isostatic pressing the powder to about 70% of the theoretical density of the alloy being used. Then the compact is encapsulated in a closely fitting aluminum alloy container or can. For degassing purposes, air is then evacuated from the can and the compact is heated to about 520.degree. C. for about 6 to 7 hours in a high vacuum. While this temperature and vacuum continue to be maintained inside the can, the canned compact is sealed and then compressed to full density at pressures above about 140 MPa. (620 MPa is generally used.)
The compact is then cooled and the container is machined away to expose uncontaminated but fully consolidated billet. Removal of the container is necessary but costly step, since the container is typically formed from an alloy that is compositional different than the powder blend used to form the billet. In addition, since the container typically buckles during compaction the final machined billet size is often greatly reduced compared to the initial diameter of the consolidated powder to insure that all of the container is removed, resulting in reduced product recovery. The billet is then heated and extruded in a conventional manner to produce a wrought product or is otherwise hot worked as by forging.
Variations of this practice are disclosed in U.S. Pat. No. 4,104,061 to S. G. Roberts. During heat-up of the compact for degassing, the container may be evacuated by vacuum, back filled with a depurative gas such as dry nitrogen, and again vacuum evacuated to facilitate the overall degassing process. In addition, the powder can be packed directly into the container, whereby the initial step of cold forming a compact is simply omitted. The hot consolidating step cannot be omitted.
Thus, in the foregoing practice and variations thereof, the use and removal of a container and the use and removal of a vacuum are seen as essential and costly steps.
It has also been known to place aluminum alloy powder in a vacuum hot press, degas the powder as placed in the press, and then hot press the powder to a near solid mass. This process is expensive because of the complex equipment used and the low production rate. Degassing is oftentimes difficult since usually only one end of the powder column is exposed to the vacuum. In vacuum hot pressing the intent is to hot press to 100% density; the result usually is a product of density 96% or more of theoretical.
It was against this background that the development of the present invention came about.
The primary object of the present invention is to provide a method for consolidating aluminum powder without the need for the use and removal of an aluminum alloy container which results in mechanical properties comparable to those produced by the above-described prior art process.
Another object of the present invention is to provide a method for consolidating aluminum powder without the need for the application of a vacuum.
Another objective of the present invention is to provide a method for consolidating aluminum powder without the need for a vacuum hot press.
These and other objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.